This invention was developed in part during work conducted under and supported by NASA research grant NAG 3-16, as well as by a grant from the University of Washington Graduate School Research Fund awarded through the Washinton Energy Research Center.
Achieving effective heat transfer is one of the principal problems facing the designers of efficient energy conversion systems for almost any application today, ranging from today's coal combustion systems to advanced fusion power and solar energy extraction systems. Ideally, a heat exchanger should be operated at temperatures matching the thermodynamic potential of the peak temperatures of the system heat source. Conventional heat exchangers, however, are rarely able to match that potential, severely limiting the capability of operating at desirable temperatures. Furthermore, when operating to recover heat at lower temperatures in a heat transfer system, large pressure drops are required by small temperature differentials, leading to correspondingly higher costs of heat transfer and concomitant inefficiencies.
Another problem in achieving efficient energy conversion systems relates to the storage of energy. Energy storage is of benefit to the operation of central power stations, for example, by providing a means of matching periodically varying loads to the uniform output desirable to operate a powerplant most efficiently. For example, an efficient and economical energy storage system must be an integral part of the design of a solar powerplant because of the intermittent nature of the solar energy resource. Few presently suggested solar power systems directly integrate energy storage into the powerplant design. Often, with the prior designs, waste heat from a primary cycle is stored, and then used at low temperatures in less efficient bottoming cycles. When energy storage is provided for use in the primary cycle, such as in a liquid sodium energy storage system, heat must still be transferred through a heat exchanger. Such storage sytems, however, are expensive. Additionally, currently available heat exchanger materials severely limit working temperatures.
High-temperature heat storage facilities and heat transfer devices are essential to economically attractive central power stations such as solar energy powerplants. For example, promising power generation cycles with efficiencies ranging from 45% to 70% can be achieved at the peak temperatures of up to 2000.degree. K. It has been suggested that capacitive heat exchangers might be employed to exploit the more efficient, higher temperature regimes because they circumvent some of the limitations of conventional tube wall heat exchangers. Capacitive heat exchangers and storage systems can use high-temperature materials and transport those materials into direct contact with a heat source. Common examples of conventional capacitive heat exchangers are packed bed regenerative heat exchangers and fluidized bed heat exchangers. However, conventional capacitive heat exchangers also suffer from serious drawbacks in that the solid phase capacitive elements, which must operate in a cyclic fashion, are subject to physical degradation resulting from thermal shock. Since the solid phase capacitive elements represent a high fraction of the total system cost of capactive heat exchangers, the replacement costs of the capacitive elements become increasingly acute as higher temperatures and larger rates of temperature change are encountered.